1. Technical Field
The present invention relates to a phosphor element which can be used as a luminescent display or various kinds of light sources used for communication or illumination, and a display device using the phosphor element.
2. Description of the Related Art
Recently, an electroluminescent (EL) element has been focused on in a flat type of display device. The EL element has characteristics such as a spontaneous illuminant property, excellent visibility, a wide viewing angle, high-speed response and the like. In addition, the EL element which has been developed at present includes an inorganic EL element using an inorganic material as an illuminant and an organic EL element using an organic material as an illuminant.
According to the inorganic EL element in which an inorganic phosphor such as zinc sulfide and the like is the illuminant, collision excitation occurs between the electrons accelerated in a high electric field of 106V/cm and the luminescent center of a fluorescent substance and when it is alleviated, light is emitted. It was found that an element having double insulation structure proposed by INOGUCHI in 1974 had high luminance and long life, and it has been put to practical use as an in-car display and the like.
The inorganic fluorescent substance is provided such that an inorganic material which becomes a luminescent center is doped in a host crystal including an insulator crystal in general. Since the host crystal is chemically stable, the inorganic EL element is highly reliable and has a long life of 30,000 hours or more. However, the electrons are not likely spread in the insulator crystal even when an electric field is applied, and it is charged because the injected electrons are accumulated on the surface. As a result, since the following injected electrons are repelled, highly energized electrons as an excitation source need to collide. Therefore, although the inorganic EL element has high reliability and a long life, it needs a high alternating voltage for driving, so that it cannot be driven in an active matrix using a thin film transistor. Consequently, it is not put to practical use as a display device in a television and the like.
In addition, according to a technique disclosed in Japanese Patent Publication No. 54-8080, emission luminance is improved by doping a transition metal element or a rare-earth element such as Mn, Cr, Tb, Eu, Tm, Yb and the like in a phosphor layer including ZnS mainly but an average luminance is less than 400 cd/m2, which is not sufficient as the display device in television and the like.
Meanwhile, according to the organic EL element in which the organic material is the illuminant, electron holes and electrons injected from electrodes form exciters and light is emitted when they transit to the ground state. A two-layer element in which an electron hole transport layer and an organic phosphor layer are sequentially stacked, which was proposed by Tang etc. in Applied Physics Letters, 51, 1987, P913, can emit light whose luminance is 1,000 cd/m2 or more at a driving voltage of 10V or less. This triggers active research and development for the organic EL element up to now.
The organic EL element which is studied at present in general will be described with reference to FIG. 5. An organic EL element 50 is formed such that a transparent electron hole injection electrode 52, an electron hole transport layer 53, a phosphor layer 56 and an electron injection electrode 58 are sequentially stacked on a transparent substrate 51. In addition, an electron hole injection layer may be provided between the electron hole injection electrode 52 and the electron hole transport layer 53, an electron transport layer may be provided between the phosphor layer 56 and the electron injection electrode 58, an electron hole block layer may be provided between the phosphor layer 56 and the electron transport layer, or an electron injection layer may be provided between the electron transport layer and the electron injection electrode 58.
The electron hole injection electrode includes ITO (indium tin oxide) film and the like which is a transparent conductive film. In order to improve transparency or lower resistivity in the ITO film, it is formed by a sputtering method, an electron beam evaporation method, an ion plating method and the like.
The electron hole transport layer is formed of a diamine derivative used by Tang etc. such as N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TRD) and the like. The material is excellent in transparency in general and it is almost transparent even when its film thickness is 80 nm.
The phosphor layer is formed of an electron transporting luminescent material such as tris(8-quinolinolato) aluminum (Alq3) and the like so as to be several tens of nm in thickness by vacuum evaporation similar to the report by Tang etc. in general. In order to implement various luminescent colors, a so-called double hetero structure in which the phosphor layer is relatively thin and the electron transport layer is laminated by about 20 nm may be employed.
As the electron injection electrode, alloy of metal having a low work function and a low electron injection barrier and stable metal having a relatively great work function, such as a MgAg alloy or an AlLi alloy proposed by Tang etc., or a laminated electrode of various electron injection layers of LiF and Al and the like is used in many cases.
In addition, an organic EL display device using a low-temperature polysilicon thin film transistor in driving each pixel is described in Journal of the Society for Information Display, vol. 8, No. 2, p93-97.
According to the conventional organic EL element, there is a defect such that a molecular bonding of the organic material which becomes the illuminant is cut because of an impressed electric field or light irradiation and the like under existence of water or oxygen, so that luminescent performance is lowered. Therefore, it cannot be practically used in continuous driving or it does not have good keeping quality. To solve the above problem, as disclosed in Japanese Patent Laid-open Publication No. 2003-59665, a hybrid organic EL element using a high-reliable inorganic material as the illuminant has been proposed.